Process for fabricating resistive memory cells

ABSTRACT

A oxide-based direct-access resistive nonvolatile memory may include within the interconnect portion of the integrated circuit a memory plane including capacitive memory cells extending in orthogonal first and second directions and each including a first electrode, a dielectric region and a second electrode. The memory plane may include conductive pads of square or rectangular shape forming the first electrodes. The stack of the dielectric layer and the second conductive layer covers the pads in the first direction and forms, in the second direction, conductive bands extending over and between the pads. The second electrodes may be formed by zones of the second bands facing the pads.

TECHNICAL FIELD

This disclosure relates to memory devices, and more particularly, tononvolatile resistive memories.

BACKGROUND

Resistive memories (RRAM), such as oxide-based direct-access memories(OxRAM), electrolytic memories (CBRAM), or even ferromagnetic memories(FRAM), have many advantages and attributes. These include very shortread and write times, low operating voltages, low power consumption,easy integration, an almost infinite endurance and a potentially veryhigh density.

RRAM resistive memories typically include memory points capable ofstoring one bit, which points are distributed in rows and columns in amatrix array in a memory plane. A memory point is accessed via wordlines traversing the rows of the memory plane and bit lines traversingthe columns of the memory plane.

In RRAM resistive memories, each memory point usually includes acapacitive metal-oxide-metal (MOM) structure or memory cell. The metallayers of the capacitive MOM structure form top and bottom electrodes,which are placed on either side of a dielectric layer and may be metaloxide, for example.

Since they include oxide and metals, the capacitive memory cells mayadvantageously be fabricated in the interconnect portion of anintegrated circuit. This portion is located above the substrate andusually designated in the art by the acronym BEOL (back-end of line).

The processes for fabricating a RRAM resistive memory utilizeconventional photolithography steps, in which a layer of photoresistdeposited on the structure being formed is irradiated in a desiredpattern. Next, the resist that was irradiated (or that was not radiated)is removed to form a resist mask to etch the exposed portion of thestructure through the resist mask.

FIG. 1 illustrates a conventional resist mask deposited on aresistive-memory memory plane PM during the manufacturing process, asseen from above. The resist mask includes “pads” 11 of square shape,periodically repeated in the directions of the rows X and the columns Yof the memory plane PM. The pads 11 eventually define the dimensions ofthe capacitive cells CEL.

In the photolithography steps, the projected images appear withirregularities, such as rounded corners. Furthermore, despite theimplementation of optical proximity corrections (OPCs), the squareportions of the resist have a tendency to round at their corners and toend up becoming circular, as shown by the dotted lines 12. Therefore,the area on which the resist pads rest decreases.

The smaller the area, the more the adhesion of the resist becomesproblematic, and the greater the risk that debonding will generatesevere fabrication defects. Thus, the densification of RRAM typememories may be limited by this problem of adhesion of the photoresist.

SUMMARY

Therefore, according to an example method of implementation, a processis provided for forming a RRAM resistive-memory memory cell or memoryplane, allowing the size of the memory points to be decreased withoutthe above-noted adhesion problem. This makes it possible to increase thedensity of the RRAM memory and also to control the aspect ratio of thecells produced.

According to one aspect, a process is proposed for producing at leastone capacitive memory cell including a first electrode and a secondelectrode separated by a dielectric region, within an interconnectportion of an integrated circuit.

According to one example aspect, the process may include a first etchingstep in which, in a first conductive layer, a first band extending in afirst direction is formed. The method further includes forming, on theetched first conductive layer, a dielectric layer and a secondconductive layer. In a second etching step, in the second conductivelayer, the dielectric layer and the etched first conductive layer, asecond band extending in a second direction orthogonal to the firstdirection is formed. The first electrode may be formed by theintersection, in the first conductive layer, of the first band and thesecond band. The second electrode may be formed by the zone or portionof the second conductive layer facing the first electrode.

According to one example implementation applicable to the production ofa resistive-memory memory plane, within the interconnect portion of theintegrated circuit, a plurality of capacitive memory cells are formed.The formation of the first electrodes of the memory cells may includethe first etching step in which, in the first conductive layer, firstbands extending in the first direction are etched and the second etchingstep in which, in the second conductive layer, the dielectric layer andthe etched first conductive layer, second bands extending in the seconddirection are etched. The second electrodes of the memory cells may beformed by zones of the second bands facing the first electrodes.

This process, which uses bands to form one or more memory cells, makesit possible to preserve the aspect ratio of the generally square orrectangular memory cell(s).

According to one example implementation, the first bands and the secondbands may be periodically distributed in the memory plane with a regularpitch in each of the two directions.

In accordance with another example aspect, the first and second etchingsteps may include depositing a photoresist followed by aphotolithography step. The resist masks rest on larger areas than inconventional methods of implementation. Thus, this aspect responds tothe problem of adhesion of the resist and advantageously allows thedensity of RRAM resistive memories to be increased.

The process may further include forming word lines traversing the memoryplane in the first direction and bit lines traversing the memory planein the second direction, forming first electrically conductive contactsconnecting the word lines to the first electrodes, and forming secondelectrically conductive contacts connecting the bit lines to the secondelectrodes.

According to another aspect, a memory device is provided which mayinclude, within an interconnect portion of an integrated circuit, atleast one capacitive memory cell including a first electrode and asecond electrode separated by a dielectric region. According to oneaspect, the first electrode may comprise a conductive pad of square orrectangular shape, and the device may further include a stack of adielectric layer and a conductive layer forming a band extending overand on each side of the pad. The second electrode may be formed by thezone of the second conductive layer facing the pad.

According to one example embodiment, the memory device may include,within the interconnect portion of the integrated circuit, a memoryplane including capacitive memory cells extending in orthogonal firstand second directions and each including a first electrode, a dielectricregion and a second electrode. The memory plane may include conductivepads of square or rectangular shape forming the first electrodes. Thestack of the dielectric layer and the second conductive layer may coverthe pads in the first direction and form, in the second direction,conductive bands extending over and between the pads. The secondelectrodes may be formed by zones of the second bands facing the pads.

According to one example embodiment, the memory may further include wordlines traversing the memory plane in the first direction and bit linestraversing the memory plane in the second direction, first electricallyconductive contacts connecting the word lines to the first electrodes,and second electrically conductive contacts connecting the bit lines tothe second electrodes.

BRIEF DESCRIPTION OF THE DRAWINGS

Other advantages and features will become apparent upon examining indetail completely nonlimiting embodiments and methods of implementation,and the appended drawings, in which:

FIG. 1 illustrates a resist mask used in conventional processes forforming resistive memory;

FIGS. 2, 4, 6 and 7 show cross-sectional views X and Y of structuresobtained in various steps for forming an RRAM resistive-memory memoryplane in accordance with an example embodiment, in planes parallel to afirst direction X and to a second direction Y that are, for example,respectively longitudinal and traverse (the directions X, Y for examplerespectively correspond to the future rows and columns of the memoryplane PM); and

FIGS. 3 and 5 are top views of the resist masks deposited on a memoryplane during RRAM resistive-memory formation in accordance with theexample embodiment, in which the first and second directions X and Yhave been shown.

DETAILED DESCRIPTION

The cross-sectional views X and Y in FIG. 2 show an example BEOLinterconnect structure including a metallization level Mi from whichcapacitive memory cells of the memory plane will be formed.

The BEOL interconnect portion is generally formed above an electroniccircuit fabricated in and on a semiconductor substrate and includes aplurality of successive metallization levels. The resistive memory cellsare, for example, formed between two metallization levels Mi and Mi+1.The metallization level Mi is shown very schematically and includesmetal tracks forming word lines WL extending in the first direction X.

In an initial step, first electrically conductive contacts CWL, whichmay be formed in a conventional way, are connected to the word lines andhave been formed in a dielectric layer OX deposited above themetallization level Mi. The surface S of the dielectric OX typicallyincludes the contacts CWL which are planarized, such as by wetchemical-mechanical planarization, for example.

In accordance with an example process for forming a RRAMresistive-memory memory plane, a first conductive layer CC1 (which willeventually serve to form the first electrodes BE of the MOM capacitivestructures) is deposited on the surface S. The first electrodes BE arethe lower electrodes of the MOM capacitive structure, i.e., theelectrodes closest to the substrate of the integrated circuit. The metalused to form the bottom electrodes BE may be chosen from titanium Ti,titanium nitride TiN, or a noble metal such as platinum Pt or iridiumIr, for example.

In a following step, a layer of photoresist is deposited on the firstlayer CC1 and subjected to a conventional photolithography and etchingphase to form longitudinal bands of resist RX. FIG. 3 shows the bands ofresist RX extending parallel to the direction X and distributedperiodically in a direction perpendicular to the direction X. The bandsof resist, which are of the same width as one another, are repeated witha regular pitch and are placed facing contacts CWL.

Next, the exposed portion of the first conductive layer CC1 is etchedselectively relative to the resist RX and as far as the surface of thedielectric OX to obtain, after removal of the resist, bands BDXextending in the first direction X of the future memory plane (FIG. 4).During this step, no problem with resist adhesion is encountered. Morespecifically, the resist pattern takes the form of bands having acontact area which is larger than for a pattern made up of pads, anddoes not include corners that risk being rounded. The resist is thenselectively removed to clear the structure obtained for implementing thesubsequent steps of the process.

FIG. 4 shows a subsequent step of the process, in which a dielectriclayer MOX and then a second conductive layer CC2 are deposited. Thesecond conductive layer CC2 will eventually include the secondelectrodes TE of the capacitive cells CEL, and may also be formed fromTi, TiN or Pt. The second electrodes TE are the upper electrodes of thecapacitive cells CEL, i.e., the electrodes furthest from the substrate.The dielectric layer MOX is advantageously a metal oxide, e.g.,including titanium oxide TiO_(x) or hafnium dioxide HfO₂.

In a following step of the process, a photoresist layer is once againdeposited and subjected to a photolithography and etching phase to formtransverse bands of resist RY perpendicular to the longitudinal bands RXobtained beforehand. FIG. 5 shows the bands of resist RY on the surfaceof the structure, including a stack of the dielectric layer MOX and thesecond conductive layer CC2 on the first conductive layer CC1 and on thesurface S of the dielectric layer OX.

The bands of resist RY extend parallel to the axis Y and areperiodically distributed in the direction X. The bands of resist RY arealso of the same width as one another, repeated with a regular pitch andplaced facing contacts CWL.

In a next step, the results of which are shown in FIG. 6, the secondconductive layer CC2, the dielectric layer MOX and the first conductivelayer CC1 are etched in succession and selectively relative to theresist as far as the surface S of the oxide layer OX. Thus, the etchingof the first conductive layer CC1 through two perpendicular masks RX, RYresults in first electrodes BE of square or rectangular shape, withoutrounding of the corners.

The dielectric layer MOX and the second conductive layer CC2 have aconfiguration in bands BDY, corresponding to the pattern of the mask RY,and form chevron-like shapes the teeth of which facing the firstelectrodes BE form the second electrodes (or upper electrodes) TE of thememory cells.

As described below with reference to FIG. 7, the second electrodes TE ofthe memory cells belonging to a given column will eventually beconnected together by a bit line. The aspect ratio of the MOM capacitivecells is thus mainly defined by the shape of the pads forming the firstelectrodes BE. Since the pads are obtained by etching perpendicularbands, this process allows such an aspect ratio to be preserved forsmall dimensions.

As FIG. 7 shows, bit lines BL extending in the transverse direction Yare formed in a conventional way, forming the columns of the memoryplane PM. The bit lines are connected to each top electrode TE of acolumn of the memory plane PM by second contacts CBL.

The bit lines BL may, for example, be produced in the uppermetallization level Mi+1. The second contacts CBL are thus producedbetween the metallization level Mi+1 and the second electrodes of thecapacitive cells CEL. For the sake of clarity, no layer of insulatingmaterial, customarily placed between the capacitive structures CEL andbetween the contacts CBL, has been shown.

Thus, a memory plane including conductive pads of a square or rectangleshape forming the first electrodes BE has been obtained. A stack of adielectric layer MOX and a second conductive layer covers the pads BE inthe first direction X and forms in the second direction Y conductivebands BDY extending over the pads and between the pads. The secondelectrodes TE are thus formed by zones of the second bands BDY,vertically facing the pads. Of course, the memory device may alsoinclude conventional selection transistors for selecting memory cells,which are not shown here for the sake of simplicity.

The methods of implementation and embodiments of the invention are notlimited to the present description but include other embodiments. Forexample, a process for producing a memory plane has been detailed, butthe invention may also be applied to the production of a single isolatedmemory cell. Those skilled in the art will be able to adapt theteachings of the present description to implement such an embodiment.

1. A device, comprising: a plurality of first electrodes arranged in aplurality of rows extending along a first direction and a plurality ofcolumns extending along a second direction that is transverse to thefirst direction; a plurality of first dielectric layers extending alongthe second direction, each of the first dielectric layers extending overa respective column of the first electrodes; and a plurality of secondelectrodes extending along the second direction, each of the secondelectrodes extending over a respective one of the plurality of firstdielectric layers.
 2. The device of claim 1 wherein the first dielectriclayers include at least one of titanium oxide or hafnium dioxide.
 3. Thedevice of claim 1, further comprising: a plurality of word linesextending along the first direction, wherein each of the firstelectrodes of a respective row is electrically coupled to same one ofthe plurality of word lines.
 4. The device of claim 3 wherein each ofthe first electrodes of a respective column is electrically coupled to adifferent one of the plurality of word lines.
 5. The device of claim 3,further comprising: a second dielectric layer on the plurality of wordlines; and a plurality of first conductive contacts extending throughthe second dielectric layer, the first electrodes being electricallycoupled to the word lines by the first conductive contacts.
 6. Thedevice of claim 5, further comprising: a plurality of bit linesextending along the second direction; and a plurality of secondconductive contacts, the second electrodes being electrically coupled tothe bit lines by the second conductive contacts.
 7. The device of claim6 wherein each of the bit lines is electrically coupled to acorresponding second electrode by two or more of the second conductivecontacts.
 8. The device of claim 1 wherein the first electrodes includeat least one of titanium (Ti), titanium nitride (TiN), platinum (Pt), oriridium (Ir).
 9. The device of claim 1 wherein the second electrodesinclude at least one of titanium (Ti), titanium nitride (TiN), platinum(Pt), or iridium (Ir).
 10. The device of claim 1 wherein the seconddirection is orthogonal to the first direction.
 11. A memory device,comprising: a substrate; a first conductive layer including a firstelectrode pad and a second electrode pad separated from the firstelectrode pad, the first and second electrode pads each having a firstside on the substrate, the first sides of each of the first and secondelectrode pads being rectangular; a band covering the first and secondelectrode pads, the band including a dielectric layer and a secondconductive layer on the first and second electrode pads; a first memorycell including the first electrode pad, the dielectric layer, and thesecond conductive layer; and a second memory cell including the secondelectrode pad, the dielectric layer, and the second conductive layer.12. The memory device of claim 11 wherein the first and second memorycells define a memory plane within an interconnect portion of the memorydevice, the memory plane including a plurality of memory cells extendingin orthogonal first and second directions.
 13. The memory device ofclaim 12 wherein the band covers the first and second electrode pads inthe first direction.
 14. The memory device of claim 12, furthercomprising: word lines traversing the memory plane in the firstdirection; bit lines traversing the memory plane in the seconddirection; first electrically conductive contacts connecting the wordlines to the first and second electrode pads; and a second electricallyconductive contact connecting one of the bit lines to the secondconductive layer.
 15. The memory device of claim 11 wherein thedielectric layer has a first side and a second side opposite the firstside, the dielectric layer having a first thickness between the firstand second sides of the dielectric layer at the first electrode pad anda second thickness between the first and second sides of the dielectriclayer at a portion of the dielectric layer between the first and secondelectrode pads, the second thickness being different from the firstthickness.
 16. The memory device of claim 11 wherein the secondconductive layer has a first side and a second side opposite the firstside, the second conductive layer having a first thickness between thefirst and second sides of the second conductive layer at the firstelectrode pad and a second thickness between the first and second sidesof the second conductive layer at the portion of the dielectric layerbetween the first and second electrode pads, the second thickness beingdifferent from the first thickness.
 17. An integrated circuit,comprising: a memory device having a first memory cell and a secondmemory cell, the memory device including: a first electrode pad; asecond electrode pad spaced laterally apart from the first electrodepad; a dielectric layer covering the first and second electrode pads;and a third electrode on the dielectric layer and overlying the firstand second electrode pads, wherein the first memory cell includes thefirst electrode, the dielectric layer, and the third electrode, and thesecond memory cell includes the second electrode, the dielectric layer,and the third electrode.
 18. The integrated circuit of claim 17 whereinthe first and second memory cells define a memory plane within aninterconnect portion of the integrated circuit, the memory planeincluding a plurality of memory cells extending in orthogonal first andsecond directions.
 19. The integrated circuit of claim 18 wherein thedielectric layer extends along the first direction.
 20. The integratedcircuit of claim 18, further comprising: word lines traversing thememory plane in the first direction; bit lines traversing the memoryplane in the second direction; first electrically conductive contactsconnecting the word lines to the first and second electrodes; and asecond electrically conductive contact connecting one of the bit linesto the third electrode.